Electron lens system



Patented Feb. 20, 1945v Unirse A STATES waffen'r1 'trios vELEo'rRcN LENS `srs'rEM Edward G. Ramberg, Powellsville, Md., asvsignor to Radio Corporation of America, a corporation of Delaware lApplication March 26, 1943, Serial No. 480,649

i 4 Claims.

' Attempts to use permanent magnets vfor the Y excitation-of electron lenses have heretofore not met withv any substantial degree of success, principally because of the leakage elds which exist f beyond the lens region. The effect of such leakage fields may be reduced by using suitablydisposed shields, but cannot be eliminated when the lenses are of uniform polarity (magnetic field direction) since, for symmetric elds produced only by permanent magnets, the integral of the field (fl-luz) along the axis must vanish; thus, elds of opposite polaritymust exist beyond the lens region. kThese fields lare objectionable because they tend to reduce the magnication of the system. Thus, as shownin Figure l, a ray r leaving n the objective lens O at a point off the axis, does not proceed, after leaving the projector lens P,

In the embodiment of the invention which has been selected for purposes of illustration the lens systemcomprises an objective lens O and a projector lens P formed by apertured pole pieces l, 2 and 3, 4 of conventional shape arranged along a common axis and excited by permanent magnets 5 and 6. Theparts 5 and B in the instant case comprise two ring magnets arranged about the upper and lower lenses O and P respectively,

but may comprise two sets of rod or bar magnets, similarly disposed. Theouter lens elements or. pole pieces l and 3 are connected to or form part of a cylinder l which is constituted of soft iron and the inner pole pieces 2 and 4 are connected to another magnetically permeable member 8 which is supported between the adjacent ends of the permanent magnetsv 5y and,v B. It will be observed that, as indicated by the symbols N (north) andS (south), the magnets 5 andB are arranged with their poles in bucking relation, hence the fields in the two lenses O and P are also ofopposite polarity. This magnetic condition in the t'wo 'lenses results in low rotational or` anisotropic aberrations since the aberrationV components due to the two lenses are of opposite sign. Since the surrounding soft iron armature 'l is connected to the upper pole piece I of the lens O and to the e lower pole piece 3 of the lens P, as well as to the in a straightr line but is bent'toward the axis, as

indicated at r', and strikes the image screen at a smaller distance from the axis than it would if the leakage field were not present.

Accordingly, the principal object of the present invention is to provide an electro-n'lens system, utilizing permanent magnets, which is free fro-m f external leakage` elds (which tend to` reduce the magnification of the instrument) and one y possessinggfurther advantages, e. g.,` from the similar poles N of the magnets, the space outside of the system is made magnetically equipotential, so that no leakage fields can there exist. p

From the foregoing it will be apparent that the present invention provides an improved permanent magnet electron-lens system which possesses advantages from the point of view of low rotational aberration and one free from external leakage fields which tend to reduce the magnification of the electron microscope or other instrument in which it is incorpo-rated.

" What is claimed is:

1. An electron lens system including, in combination, a plurality of permanent magnets arranged with their poles in bucking relation, a plurality of pairs of apertured lens velements mounted, respectively, in the magnetic path between the opposite poles of each magnet, and a magnetically permeable structure connecting the like poles of said magnets one to another,

2. The invention' as set forth in claim 1 and wherein said magnetically permeable structure comprises a support for said plurality of permanent magnets. y

3. An electron lens system comprising, in combination, a pair of permanent magnets arranged with their poles in bucking relation, a magnetically permeable member including a. plurality of oppositely extending apertured lens elements supported between the adjacent ends of said permanent magnets, and a. second magnetically permeable member connected to the remote ends of said permanent magnets and including a plurality of apertured lens elements arranged in register with said first-mentioned apertured lens elements.

4. The invention as set forth in claim 3 and wherein said second-mentioned magnetically permeable member comprises a hollow casing Within which said permanent magnets and said mst-mentioned magnetically permeable member are mounted.

EDWARD G. BAMBERG. 

